Ink jet recording is, as well known, a process in which an ink jet recording ink is ejected as droplets through minute nozzles to form many dots on the surface of a recording medium, thereby recording images such as letters or figures on paper, etc.
As such ink jet recording, there have been developed and put to practical use a process in which an electric signal is converted to a mechanical signal by the use of an electrostrictive element to intermittently ejecting an ink stored in a nozzle head section, thereby recording letters or symbols on the surface of a recording medium; and a process in which an ink stored in a nozzle head section is rapidly heated at a portion very close to an ejection portion to generate bubbles, and intermittently ejected by volume expansion due to the bubbles to record letters or symbols on a surface of a recording medium.
According to the ink jet recording, it is possible to print high-resolution, high-quality images at high speed by an easy way. In particular, the ink jet recording has recently become an image formation method substitutable for a photographic method in color printing.
As inks that can be used in the ink jet recording, there have been widely known an aqueous dye ink containing a dye as a colorant and an aqueous pigment ink containing a pigment as a colorant. The aqueous pigment ink has the advantage that printed matter excellent in weather resistance can be prepared. However, when used alone, the pigment is difficult to dissolve in an aqueous solvent. Accordingly, the aqueous pigment ink usually contains a polymer type dispersing agent for dispersing the pigment at a specified concentration.
Meanwhile, for example, when photographic tone images are reproduced by the ink jet recording, the ink jet recording is usually carried out so as to increase the density of dots for a region having high print density and to decrease the density of dots for a region having low print density.
However, particularly, the use of the above-mentioned aqueous pigment ink as the ink results in the presence of the aqueous pigment ink in the region in which the density of dots is increased (high duty region) in larger amounts, compared to that in the region in which the density of dots is decreased (low duty region), and accordingly results in the presence of the polymer type dispersing agent in larger amounts. The polymer type dispersing agent usually has glossiness, so that the high duty region of printed matter shows a high gloss, and the low duty region shows a low gloss. Accordingly, there has been the problem that when the printed matter is observed as a whole, uneven gloss becomes obvious depending on the contents of an image to be reproduced.
In order to solve this problem, there has been known a technique of further printing the above-mentioned printed matter with a clear ink capable of imparting glossiness in such amounts that the amount thereof in the low duty region is larger than that in the high duty region, thereby intending to decrease the difference in glossiness between the high duty region and the low duty region and to solve the problem of the above-mentioned uneven gloss.
On the other hand, in order to maintain the quality of images obtained by the ink jet recording, the absence of dot omission is required. The term “dot omission” means the phenomenon that no dot is formed at a position where the dot is originally to be formed because of, for example, troubles in nozzles of a recording head mounted on an ink jet recording apparatus.
As a method for detecting the dot omission, there has been known a method, as shown in FIG. 5, of providing a light emitting unit 113 for emitting light L and a light receiving unit 114 for receiving the above-mentioned light L so that an ink 111 passes therebetween from a recording head 110 to paper 112, subsequently, continuously flying the ink 111 at predetermined intervals, thereby forming a detection pulse based on intermittent interception of the above-mentioned light L with the above-mentioned ink 111, and comparing the pulse interval of the detection pulse with a specified threshold value (see patent document 1).
Patent Document: JP 2002-192740 A
However, the method described in patent document 1 detects the dot omission by inspecting whether the light L emitted from the light emitting unit 113 is intercepted with the ink 111 at predetermined timing or not, when the ink 111 is allowed to drop from the recording head 110 at proper timing. Accordingly, when a clear ink or a reactive clear ink is used, it has been difficult to detect the dot omission of the clear ink or the reactive clear ink, because the light L is difficult to be intercepted therewith.